Semiconductor laser

ABSTRACT

A double heterojunction semiconductor laser according to the invention includes first and third cladding layers sandwiching an active layer. The third cladding layer includes a mesa opposite a light emitting region of the active layer. The mesa is confined by a current blocking layer. A cap layer that is part of the mesa is used as a dopant diffusion source to dope a light emitting region of the active layer heavily. A second cladding layer may be present between the active layer and third cladding layer having the same conductivity type as the third cladding layer adjacent the light emitting region but the opposite conductivity type elsewhere. A semiconductor laser according to the invention may also include a stripe groove structure. The semiconductor lasers include pnpn structures outside the light emitting region and in window structures adjacent the facets of the semiconductor laser for suppressing leakage currents, thereby increasing laser efficiency and reducing threshold current while increasing power output.

This application is a division of application Ser. No. 586,197, filedSept. 20,1990.

FIELD OF THE INVENTION

The present invention relates to a double heterojunction semiconductorlaser having increased power output, reduced current leakage, andimproved efficiency.

BACKGROUND OF THE INVENTION

In double heterojunction semiconductor lasers, an active layer in whichcarrier recombination occurs, resulting in the emission of light, issandwiched between cladding layers of opposite conductivity type. Thecladding layers have larger energy band gaps and smaller refractiveindices than the active layer in order to confine light within theactive layer. The laser structure includes two opposed, generallyparallel facets that are generally perpendicular to the active layer.The facets are coated with a reflective material to produce, with theactive layer, an optical resonator in which light resonates to sustain alaser oscillation. The coating on at least one of the facets permitssome of the laser light to escape, producing the light output of thelaser.

A number of factors limit the power output of a semiconductor laser.Carrier recombination can occur more efficiently at the surfacesadjacent the facets than within the body of the laser. The increasedcarrier recombination and resulting increased charge carrier density atthe facets results in increased light absorption there. That lightabsorption, in turn, increases the temperature at the facets. If thetemperature rise is sufficient, localized melting of the semiconductormaterials can occur, resulting in catastrophic optical damage (COD) thatdestroys the laser.

The power output of a semiconductor laser can be increased withoutrisking COD by providing a window structure as described by Yonezu et alin the Journal of Quantum Electronics, Volume QE-15, August 1979, pages775-781, the disclosure of which is incorporated herein by reference. Inthe window structure described by Yonezu, the regions of thesemiconductor laser adjacent the facets, i.e., the windows, are heavilydoped n-type and the light emitting region, which lies between thewindows in the central portion of the laser, is made p-type byovercompensation with a p-type dopant. As a result of this dopingprofile, the energy band gap in the central portion of the laser isdecreased relative to the energy band gap in the windows. The increasedenergy band gap in the window structures results in reduced absorptionof light near the facets, thereby increasing the power level that can beattained without risk of COD.

Although the window structure increases the power output that can besafely produced by a laser, the relatively high doping concentrationsassociated with the window structure create other problems. For example,when the dopant concentration is relatively high in the light emittingregion where carriers recombine and emit light, there is significantlight loss due to free carrier absorption, i.e., the absorption of lightby charge carriers. In addition, the relatively heavy dopantconcentrations throughout the laser structure encourage the flow ofleakage currents between the laser electrodes which are generallyparallel to the active layer and transverse to the facets. These leakagecurrents reduce laser efficiency and effectively raise the currentthreshold at which laser oscillation begins.

SUMMARY OF THE INVENTION

The present invention is directed to solving the problems of internallight absorption in a double heterojunction semiconductor laserincorporating a window structure and to reducing current leakage in adouble heterojunction semiconductor laser, particularly a laserincorporating a window structure.

According to a first aspect of the invention, a semiconductor laserincludes a semiconductor substrate of a first conductivity type, asemiconductor first cladding layer of the first conductivity typedisposed on the substrate, a semiconductor active layer disposed on thefirst cladding layer and having a central light emitting region of asecond conductivity type opposite the first conductivity type, asemiconductor third cladding layer of the second conductivity typedisposed on the active layer including a mesa opposite and projectingaway from the light emitting region of the active layer, a semiconductorcurrent blocking layer of the first conductivity type disposed on thethird cladding layer and adjacent the mesa, a semiconductor fourthcladding layer of the second conductivity type disposed on the currentblocking layer and on the mesa, a semiconductor contacting layer of thesecond conductivity type disposed on the fourth cladding layer, andfirst and second electrodes respectively disposed on the substrate andthe contacting layer wherein the laser includes generally parallel firstand second facets transverse to the first and second electrodes fortransmitting laser light outside the laser and a semiconductor cap layerin the mesa adjacent the fourth cladding layer, the cap layer having thefirst conductivity type proximate the first and second facets and thesecond conductivity type elsewhere.

A method of manufacturing a semiconductor laser according to theinvention includes successively growing a semiconductor first claddinglayer of the first conductivity type, a semiconductor active layer, asemiconductor third cladding layer of a second conductivity typeopposite the first conductivity type, and a semiconductor cap layer ofthe first conductivity type on a semiconductor substrate of a firstconductivity type, diffusing a dopant producing the second conductivitytype into the cap layer except at portions where each of two opposedfacets of the semiconductor layer will be formed, thereby converting thecap layer in the diffused portion to the second conductivity type,removing portions of the third cladding layer and the cap layer to leavea mesa including portions of the third cladding layer and the cap layerprojecting from a remaining portion of the third cladding layer, heatingthe substrate, first cladding layer, active layer, and mesa to diffusethe dopant from the cap layer through the mesa and into the active andthird cladding layers adjacent the mesa, growing a semiconductor firstconductivity type current blocking layer on the third cladding layerabutting the mesa, successively growing a semiconductor fourth claddinglayer of the second conductivity type and a semiconductor contactinglayer of the second conductivity type on the current blocking and caplayers, depositing first and second electrodes on the substrate and thecontacting layer, respectively, and forming a pair of generally parallelopposed facets generally transverse to the first and second electrodesand spaced from the portions of the cap layer into which the dopant wasdiffused.

According to another aspect of the invention, a semiconductor laserincludes a semiconductor substrate of a first conductivity type, asemiconductor first cladding layer of the first conductivity typedisposed on the substrate, a semiconductor active layer disposed on thefirst cladding layer and having a central light emitting region of asecond conductivity type opposite the first conductivity type, asemiconductor third cladding layer disposed on the active layer, asemiconductor current blocking layer of the first conductivity typedisposed on the third cladding layer, the current blocking layerincluding an opening extending to the third cladding layer, asemiconductor fourth cladding layer of the second conductivity typedisposed on the current blocking layer and on the third cladding layerin the opening in the current blocking layer, a semiconductor contactinglayer of the second conductivity type disposed on the fourth claddinglayer, and first and second electrodes respectively disposed on thesubstrate and the contacting layer wherein the laser includes generallyparallel first and second facets transverse to the first and secondelectrodes for transmitting laser light outside the laser, the thirdcladding layer is of the second conductivity type opposite the lightemitting region of the active layer and of the first conductivity typeproximate the facets and elsewhere outside the light emitting region ofthe active layer.

Another method of manufacturing a semiconductor laser according to theinvention includes successively growing a semiconductor first claddinglayer of the first conductivity type, a semiconductor active layer, asemiconductor second cladding layer of the first conductivity type, asemiconductor third cladding layer of a second conductivity typeopposite the first conductivity type, and a semiconductor currentblocking layer of the first conductivity type on a semiconductorsubstrate of a first conductivity type, forming a diffusion maskincluding a central opening on the current blocking layer, the maskcovering portions of the semiconductor layers proximate the locationswhere facets of the laser will be formed, diffusing a dopant producingthe second conductivity type through the opening in the diffusion maskinto the current blocking layer, heating the substrate, first claddinglayer, active layer, second and third cladding layers, and currentblocking layer to diffuse the dopant from the third cladding layerthrough the second cladding layer and into the active layer, removingthe diffusion mask and depositing on the current blocking layer anetching mask having a central opening extending to the locations wherethe facets of the semiconductor laser will be formed and aligned withthe location of the opening of the diffusion mask, removing the portionof the current blocking layer not covered by the etching mask byetching, removing the etching mask, successively growing a semiconductorfourth cladding layer of the second conductivity type and asemiconductor contacting layer of the second conductivity type on thecurrent blocking layer and on the third cladding layer where a portionof the current blocking layer was removed, depositing first and secondelectrodes on the substrate and the contacting layer, respectively, andforming a pair of generally parallel opposed facets generally transverseto the first and second electrodes spaced from the portions of thesecond cladding layer into which the dopant was diffused.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter. The detaileddescription is given by way of illustration only, since variousadditions and modifications within the spirit and scope of the inventionwill be apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view of a semiconductor laser according tothe invention with the central portion of the laser shown incross-section;

FIG. 1(b) is a sectional side view taken along line 1(b)--1(b) of FIG.1(a);

FIGS. 2(a)-2(d) are central cross-sectional views illustrating steps ina method for manufacturing the semiconductor laser of FIG. 1(a);

FIGS. 3(a) and 3(b) are central cross-sectional views of the laserstructure of FIG. 1(a) illustrating defects that may occur duringmanufacture;

FIG. 4(a) is a perspective view of a semiconductor laser according toanother embodiment of the invention with the central portion of thelaser shown in cross-section;

FIG. 4(b) is a sectional side view taken along line 4(b)--4(b) of FIG.4(a);

FIG. 5(a) is a sectional side view of a precursor for making a laserhaving the structure shown in FIG. 4(a);

FIGS. 5(b)-5(g) are central cross-sectional views illustrating, inconjunction with FIG. 5(a), a method for manufacturing the laser of FIG.4(a).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A double heterojunction semiconductor laser according to an embodimentof the invention is shown in a perspective view in FIG. 1(a). In orderto illustrate the internal structure of the laser, in FIG. 1(a), thecentral portion of the laser has been separated and moved to the right,away from the end sections of the laser. The end sections include thefacets and are different from the central portion of the laser becauseof the presence of the window structures. In FIG. 1(b), a side sectionalview of the laser of FIG. 1(a) taken along line 1(b)--1(b) of FIG. 1(a)is shown to illustrate further the window structures.

In FIGS. 1(a) and 1(b), an n-type gallium arsenide substrate 1 has anumber of semiconductor layers successively disposed on it, forming thelaser structure. Those layers are a first cladding layer 2 of n-typealuminum gallium arsenide, an active layer 3 of gallium arsenide, asecond cladding layer 4 of n-type aluminum gallium arsenide, and a thirdcladding layer 5 of p-type aluminum gallium arsenide. A current blockinglayer 6 of n-type gallium arsenide is disposed on part of the thirdcladding layer.

The double heterojunction laser of FIGS. 1(a) and 1(b) has a stripegroove-type construction. A stripe groove 11 extends through the currentblocking layer 6 and exposes the third cladding layer 5 from one facet18 to the other. A fourth cladding layer 7 of p-type aluminum galliumarsenide is disposed on the current blocking layer and in the stripegroove 11 in contact with the third cladding layer 5. A contacting layer8 of p-type gallium arsenide is disposed on the fourth cladding layer 7.Electrodes 9 and 10 are respectively disposed on the substrate 1opposite the other semiconductor layers and on contacting layer 8 tocomplete the laser.

In the structure as described, three pn rectifying junctions are presentbetween the electrodes 9 and 10 along a current path through the currentblocking layer 6. One pn junction is present between the fourth claddinglayer 7 and the current blocking layer 6 where those two layers are incontact, and a second pn junction is present between the currentblocking layer 6 and the third cladding layer 5 where those two layersare in contact. Finally, a third rectifying junction is present betweenthe second and third cladding layers 4 and 5. Leakage current flows,i.e., current flows other than through the active layer 3 at the stripegroove 11, are strongly suppressed by these three pn junctions. Acurrent path between the electrodes 9 and 10 passing through the stripegroove 11 and the as deposited layers includes only one pn junctionbetween second and third cladding layers 4 and 5 that lies on the sideof the active layer toward electrode 10.

In the laser structure of FIG. 1(a), the pn junction between the secondand third cladding layers is eliminated only in the vicinity of thestripe groove 11 and only in the central portion of the laser by adopant that produces p-type conductivity. That dopant is disposed in theactive and second and third cladding layers opposite the stripe groove11 in a region 12 in FIG. 1(a). The p-type dopant overcompensates thesecond cladding layer 4, converting it to p-type conductivity in region12 opposite stripe groove 11 in the central portion of the laser.Thereby, in the central portion of the laser there exists a current pathbetween electrodes 9 and 10 in which only one pn junction, whichsandwiches the active layer 3, is present. That pn junction, whenproperly biased, produces carrier recombinations that result in laserlight generation.

As clearly shown in sectional side view in FIG. 1(b), region 12 islimited to the central portion of the laser. At each end of region 12,adjacent one of the facets 18 in a window region 13, the p-type dopantthat overcompensates second cladding layer 4 is absent from that layerand layers 3 and 5. The absence of the overcompensating p-type dopantfrom the window regions reduces current flow, surface recombination, andlight absorption so that the laser can produce higher output power,without COD, than is possible when the window structures are absent.

A method for manufacturing the laser structure of FIGS. 1(a) and 1(b) isillustrated in FIGS.

A double heterojunction semiconductor laser 2(a)-2(d). Each of thosefigures is a cross-sectional view taken in the central portion of thelaser. The diffusion steps illustrated in FIGS. 2(a) and 2(b) in whichp-type impurities are disposed in the active layer and second and thirdcladding layers do not take place in the window regions 13 adjacentfacets 18.

Turning to FIG. 2(a), the first cladding layer 2, the active layer 3,the second and third cladding layers 4 and 5, and a current blockinglayer 6 are successively grown by a known process, such as metal organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), liquidphase epitaxy (LPE), or the like. At this stage, the stripe groove 11 isnot yet formed in current blocking layer 6. Thereafter, a film 15 thatis effective as a diffusion mask against p-type impurities that will bediffused into the precursor of the laser structure is deposited on thecurrent blocking 6. When the p-type dopant is zinc, silicon nitridedeposited in a thermal chemical vapor deposition process may be employedas the film 15. After the deposition of the film 15, a central apertureis opened in it as indicated in FIG. 2(a). That aperture is not presentin the window regions. Since the laser facets are usually formed afterthe laser structure is complete, the aperture in the diffusion mask isgenerally rectangular with the short ends of the rectangle spaced fromthe locations where the laser facets 18 will be subsequently formed.

In the specific embodiment described, zinc is diffused through thediffusion mask aperture into and penetrating the current blocking layer6. The source of the zinc atoms may be a vapor or a solid phasediffusion source, such as a film containing a mixture of zinc compoundand another compound, for example, zinc oxide and silicon dioxide. Inthe illustrated method, a vapor phase zinc source is employed. Theprecursor of the laser structure, usually in a wafer form, is heated ina sealed quartz ampoule in the presence of zinc arsenide and elementalzinc to about 700° C., vaporizing the zinc. The zinc arsenide providesan excess arsenic pressure that protects the compound semiconductorlayers including arsenic from deterioration. The zinc concentration inthe current blocking layer opposite the diffusion mask aperture reachesabout 1×10²⁰ cm⁻³. The resulting high p-type dopant concentration isundesirable in the completed laser because it strongly encourages freecarrier absorption, resulting in light losses as already described. Inkeeping with achieving that goal, the time and temperature of thediffusion, considering the thickness of the current blocking layer 6,are limited so that the diffusion front of zinc in this first diffusionstep does not reach the second cladding layer 4.

In order to convert the portion of the second cladding layer 4 oppositethe aperture in the diffusion mask film 15 from n-type to p-type, zincmust be further diffused to penetrate that layer. That diffusion isaccomplished in a drive-in diffusion step. That drive-in diffusion iscarried out at a higher temperature, for example, approximately 900° C.,that the first diffusion in an arsenic atmosphere. The arsenic isnecessary to avoid deterioration of the exposed surface of the galliumarsenide current blocking layer 6. As a result of this second diffusion,some of the zinc present in the current blocking layer 6 reaches andpenetrates the second and third cladding layers 4 and 5, opposite theaperture in the diffusion mask film 15, and also enters the active layer3. The resulting zinc impurity concentration in region 12 isapproximately 10¹⁸ to 10¹⁹ cm⁻³.

In order to expose the third cladding layer for deposition of the fourthcladding layer, the zinc diffused region 14 within the current blockinglayer must be removed. The diffusion mask film 15 covers the windowregions of the structure. However, to produce the desired laserstructure, the stripe groove 11 must extend the full length of the laserresonator, i.e., completely between facets 18. Therefore, the diffusionmask is not suitable as an etching mask for forming the stripe grooveand is removed. A photoresist film 16 is deposited on current blockinglayer 6 and a central aperture is opened in it, fully extending betweenthe facet locations and aligned with the region 14 in which zinc hasbeen diffused. As shown in FIG. 2(c), the stripe groove 11 is formed byselectively etching the gallium arsenide current blocking layer 6through the aperture in the photoresist film 16 without substantiallyetching the aluminum gallium arsenide third cladding layer 5. Thereby,the most highly doped zinc diffusion region 14 that would produce freecarrier absorption and light loss in the completed laser is removed.

The laser is completed by removing the etching mask 16 and successivelygrowing the fourth cladding layer 7 and the contacting layer 8 usingMOCVD, MBE, LPE, or the like. Electrodes 9 and 10 are applied toopposite sides of the device and the facets 18 are formed, for example,by cleaving at the preselected locations to preserve the windowstructure at the opposite ends of the double heterojunctionsemiconductor laser.

The laser of FIG. 1(a) provides improved performance. When forwardbiased, relatively high power laser light can be produced by the laserwithout damage to the facets because of the window structure. Inaddition, leakage currents, i.e., currents flowing between electrodes 9and 10 other than through the active layer at the stripe groove, aresuppressed. The only current paths between the electrodes outside thestripe groove include a pnpn structure, i.e., three rectifyingjunctions, that is highly effective in concentrating the current flow inthe stripe groove region. A pn junction is present in the stripe groove11 between second third cladding layers 4 and 5 in the window regions13. That junction is electrically connected in parallel with therelatively heavily doped p-type region 12. Therefore, current flow atthe facets 18 through the stripe groove and a rectifying junction issuppressed in favor of a flow through the single conductivity typeregion 12, further discouraging current leakage. The improved currentconcentration achieved in the laser structure of FIG. 1(a) reduces thethreshold current at which laser oscillation occurs and increases laserefficiency.

In the manufacturing steps illustrated in FIGS. 2(b) and 2(c), it isimportant to align the aperture of the etching mask formed with film 16with the former location of the aperture of the diffusion mask formedwith film 15. The alignment is desirable to ensure that all of thehighly doped zinc region 14 in the current blocking layer 6 is removedwhen the stripe groove is formed. Preferably, the aperture in theetching mask is wider than the aperture in the diffusion mask to ensurethat any portion of the current blocking layer in which zinc haslaterally diffused is removed in the etching step. If these conditionsare not met, the effectiveness of the structure in concentrating thecurrent flow through the active layer only at the stripe groove isreduced.

FIGS. 3(a) and 3(b) illustrate the structures that can result whendesired alignment between the diffusion and etching masks is notachieved. In FIG. 3(a), there has been a slight misalignment between theapertures of the diffusion and etching masks. As a result, a residualportion of the highly doped zinc region 14 has been left in part of thecurrent blocking layer adjacent the stripe groove 11. A current leakagepath is thereby provided, resulting in a non-linear relationship betweenlight output and the current flowing through the laser as well asreduced efficiency. In FIG. 3(b), the aperture in the etching mask wassmaller than the aperture in the diffusion mask. As a result, residualportions of the highly doped region 14 are left at each side of thestripe groove in the current blocking layer 6. These two p-type regionsprovide a still larger current leakage path than in the structure ofFIG. 3(a), again resulting in lowered laser efficiency as well asnon-linearity between laser current and light output.

In forming the stripe groove 11 in the process step illustrated in FIG.2(c), the aluminum gallium arsenide third cladding layer 5 is exposed tothe ambient. That exposure may result in the oxidation of the exposedaluminum in the third cladding layer 5. The oxidation at the regrowthinterface may interfere with the deposition of the fourth claddinglayer. The potentially oxidized surface is positioned directly oppositethe stripe groove where current is concentrated for the laseroscillation. This close proximity of an oxidized layer to the lightemitting region may cause premature deterioration of the semiconductorlaser.

A second embodiment of the invention is shown in perspective andsectional views in FIG. 4(a) and in a side sectional view in FIG. 4(b).In this embodiment, there is no regrowth interface subject to oxidationlocated near the light emitting region or through which a concentratedcurrent flows for producing laser light. Moreover, the possibility of aleakage current path in the current blocking layer resulting frommisalignment of mask apertures is eliminated.

In FIGS. 4(a)-5(g), the same elements are given the same referencenumbers as in FIGS. 1(a)-2(d). Therefore, it is not necessary toidentify again the elements that have already been described withrespect to the other figures.

The double heterojunction laser embodiment of FIG. 4(a) employs areverse mesa structure rather than the stripe groove structure of thelaser embodiment of FIG. 1(a). The reverse mesa includes side walls thatconverge when moving in the direction from the electrode 10 toward theactive layer 3. The reverse mesa is an extension of the third claddinglayer 5 and is disposed within an opening in the current blocking layer6. The reverse mesa includes, in the central portion of the laser, arelatively highly doped region 14 containing a dopant producing p-typeconductivity. In addition, a cap layer 21 of n-type gallium arsenide inthe window regions 13, but overcompensated to p-type in the centralportion of the laser, is present at the top of the reverse mesa, i.e.,sandwiched between the mesa and the fourth cladding layer 7 and by thecurrent blocking layer 6. As in the embodiment shown in FIG. 1(a), ap-type region 12 is present in the active layer 3 in the light emittingregion in the central portion of the laser but not in the windowstructures adjacent the facets of the laser. The laser embodiment ofFIG. 4(a) includes, outside the light emitting region, both in thecentral portion cf the laser and at the facets 18, a pnpn structure thatis effective in concentrating current flow in the light emitting regionand reducing leakage currents that may flow between the electrodes 9 and10 outside that light emitting region.

A method for manufacturing the double heterojunction semiconductor laserof FIG. 4(a) is illustrated in FIGS. 5(a)-5(g). FIG. 5(a) is a sidesectional view, like FIG. 4(b), whereas FIGS. 5(b)-5(g) are transversesectional views taken in the central portion of the laser structure.Turning to FIG. 5(a), the n-type aluminum gallium arsenide firstcladding layer 2, the gallium arsenide active layer 3, the n-typealuminum gallium arsenide second cladding layer 4, and the p-typealuminum gallium arsenide third cladding layer 5 are successively grownusing a known process, such as MOCVD, MBE, or LPE. Unlike the structureillustrated in FIG. 2(a), in FIG. 5(a), the third cladding layer 5 isrelatively thick, for example, 1.5 to 2 microns thick. The increasedthickness is required for the formation of the mesa structure, asexplained below. An n-type gallium arsenide cap layer 21 is grown on thethird cladding layer 5 as the final step in the initial growth process.

After the initial growth process, as illustrated in FIG. 5(a), a p-typedopant, such as zinc, is diffused into the cap layer 21 except at thewindow regions 13 at opposite ends of the structure where facets 18 areto be formed. Although not illustrated in FIG. 5(a), a diffusion mask,such as a silicon nitride layer with a rectangular aperture exposing thecentral portion of the cap layer 21, while protecting the regions atwhich the facets 18 will be formed, is disposed on the cap layer beforethe diffusion step. The zinc dopant may be supplied in a vapor formthrough the aperture or from a solid diffusion source disposed on thecap layer 21 and produces a relatively high p-type dopant concentrationwithin the cap layer 21 except at the facet regions. Unlike the zincdiffusion mask and step described with respect to FIG. 2(a), the openingin the diffusion mask need not be limited transversely, i.e., along adirection parallel to the facets 18 and the active layer 3, to thecentral portion of the laser structure. Because the cap layer 21 issubsequently removed between the facets, except opposite the lightemitting region, it is not necessary to protect those other areas of caplayer 21 from zinc. It is only necessary to protect from zinc theregions at which the facets will be formed in order to produce thewindow structures.

Following the zinc diffusion, the diffusion mask is removed and anadditional mask 22 is applied. Mask 22 may be silicon nitride andextends longitudinally in the direction between and beyond the twofacets, i.e., to and covering the window regions 13. The silicon nitridefilm 22 is formed by a conventional process, such as a thermally drivenchemical vapor deposition process, and patterned by conventionalphotolithographic and selective etching steps. Initially, the mask 22 isemployed as an etching mask. Where not protected by etching mask 22, thethird cladding layer 5 is selectively etched to leave reverse mesa 23 inplace. As is well known in the art, this reverse mesa structure can beobtained by proper crystallographic orientation of the substrate 1 anduse of a crystallographically preferential etch. For example, thesubstrate orientation may be (100) and a reverse mesa may be formedalong the <011> direction by use of an etch that is an aqueous solutionof sulfuric acid and hydrogen peroxide. The manufacturing steps of FIGS.5(b)-5(g) and the structure shown in FIG. 4(a) employ a reverse mesastructure which is preferred However, the invention also encompasses aforward mesa structure in the <011> direction. In a forward mesa, theside walls diverge, moving in the direction from electrode 10 toward theactive layer 3.

The etching step in the formation of mesa 23 exposes surfaces ofaluminum gallium arsenide layer 5 that are subject to oxidation.However, as will be described below, the concentrated current flow thatpasses through the active layer to produce laser oscillation does notpass through those oxidized surfaces. In other words, the potentiallyoxidized surfaces are remote from the light emitting region so that theydo not produce premature deterioration of the semiconductor laser.

The relatively heavily doped region 14 provides a diffusion source for adrive-in diffusion step illustrated in FIG. 5(c). During the drive-indiffusion process, as in the initial diffusion doping region 14, anexcess pressure of arsenic is employed to avoid deterioration of thegallium arsenide and the aluminum gallium arsenide layers. During thedrive-in diffusion, zinc diffuses from region 14 under mask 22 throughmesa 23, i.e., third cladding layer 5, into second cladding layer 4 andactive layer 3. Lateral diffusion of the dopant atom is prevented by theside walls of the reverse mesa 23. As a result, the misalignmentsillustrated in FIGS. 3(a) and 3(b) between zinc diffusions and currentblocking layers cannot occur in the step illustrated in FIG. 5(c) andthe doped region 12 at the active layer 3 is precisely aligned with thebase of the reverse mesa 23. During this drive-in diffusion step, mask22 protects the top surface of the cap layer from damage ordeterioration.

Following the drive-in diffusion, as illustrated in FIG. 5(d), then-type current blocking layer 6 is grown by MOCVD on the third claddinglayer 5 including abutting the side walls of the reverse mesa 23 and onthe longitudinal surfaces of third cladding layer 5 that were exposedduring the etching step. The current blocking layer 6 is grown to asufficient height to bury the reverse mesa but not the silicon nitridefilm 22 on which there is no deposition of gallium arsenide when theMOCVD process is used.

Turning to FIG. 5(e), the silicon nitride film 22 is removed byselective etching, exposing the cap layer 21. Thereafter, as shown inFIG. 5(f), the fourth cladding layer 7 of p-type aluminum galliumarsenide and the contacting layer 8 of p-type gallium arsenide aresuccessively grown by MOCVD or another conventional process on the caplayer 21 and current blocking layer 6. Finally, metal electrodes 9 and10 are deposited on substrate 1 and contacting layer 8, respectively.

In the structure of FIGS. 4(a) and 5(g), the only regrowth interfacethrough which current producing laser oscillations flows is between caplayer 21 and fourth cladding layer 7. That interface is not susceptibleto oxidation because cap layer 21 does not contain aluminum. Thatinterface is relatively widely separated from the light emitting regionby at least the thickness of the current blocking layer 6, i.e., aboutthe same separation as the original thickness of third cladding layer 5,1.5 to 2 microns. In the stripe groove structure of FIG. 2(d), theregrowth interface between third cladding layer 5 and fourth claddinglayer 7 is separated from the active layer 3 only by the thickness ofsecond and third cladding layers 4 and 5, a much smaller distance thanin the structure of FIG. 5(g).

The operation of the structure of FIGS. 4(a) and 5(g) is essentially thesame as that of the structure of FIG. 1(a). Both structures provideincreased power output without facet damage because of the windowstructure. In addition, both structures have reduced threshold currentsfor laser oscillation and increased current efficiency because of thepnpn structure outside the light emitting region of the active layerthat suppresses leakage currents and enhances the concentration ofcurrent flow through the light emitting region of the active layer. Inaddition, in the structure of FIG. 4(a), a pnpn structure is alsopresent at each of the window regions in a current path passing throughthe reverse mesa. The n-type cap layer 21 retains its n-typeconductivity proximate the facets in the window structures since it isprotected from the diffusion step establishing the p-type region 14 andis not affected by the drive-in diffusion step. The cap layer 21produces the additional pn junctions in the window region at the mesa.Therefore, a further improvement in concentrating the current flowthrough the active layer at the light emitting region and suppressingleakage currents is achieved in the structure of FIG. 4(a) compared tothe structure of FIG. 1(a).

The described structures employ gallium arsenide as the active layer 3.However, if shorter wavelength light laser is desired, the active layer3 may be aluminum gallium arsenide. The zinc diffusion step illustratedin FIGS. 2(a) and 5(a) may employ a solid source of dopant, such as azinc oxide film or a film of zinc oxide mixed with another material,such as silicon dioxide. After the initial diffusion, a solid diffusionsource is removed by etching. Dopant atoms other than zinc producingp-type conductivity in compound semiconductors, such as cadmium,magnesium, and beryllium, may be employed in place of zinc. In addition,compound semiconductor materials other than those described here, suchas indium gallium arsenide phosphide or aluminum gallium indiumphosphide, may be employed. When these materials are used, theproportions of various components are adjusted in the semiconductorlayers to produce a double heterojunction structure including an activelayer of smaller energy band gap and larger refractive index than thecladding layers in order to obtain light confinement and laseroscillation.

In the structures described, the second cladding layer 4 is preferablyincluded because it adds a pn junction that assists in confining currentflow to the light emitting region of the active layer, reducing leakagecurrents, increasing laser efficiency, and reducing the thresholdcurrent. However, the second cladding layer 4 is not essential to everyembodiment of the invention.

We claim:
 1. A semiconductor laser comprising:a semiconductor substrateof a first conductivity type; a semiconductor first cladding layer ofthe first conductivity type disposed on said substrate; a semiconductoractive layer disposed on said first cladding layer and having a centrallight emitting region of a second conductivity type opposite the firstconductivity type; a semiconductor third cladding layer of the secondconductivity type disposed on said active layer including a mesaopposite and projecting away from the light emitting region of saidactive layer; a semiconductor current blocking layer of the firstconductivity type disposed on said third cladding layer and adjacentsaid mesa; a semiconductor fourth cladding layer of the secondconductivity type disposed on said current blocking layer and on saidmesa; a semiconductor contacting layer of the second conductivity typedisposed on said fourth cladding layer; and first and second electrodesrespectively disposed on said substrate and said contacting layerwherein said laser includes generally parallel first and second facetstransverse to said first and second electrodes for transmitting laserlight outside said laser and a semiconductor cap layer in said mesaadjacent said fourth cladding layer, said cap layer having the firstconductivity type proximate said first and second facets and the secondconductivity type elsewhere.
 2. The semiconductor laser of claim 1including a semiconductor second cladding layer interposed between saidactive layer and said third cladding layer, said second cladding layerbeing of the second conductivity type opposite the light emitting regionof said active layer and of the first conductivity type elsewhere. 3.The semiconductor laser of claim 1 wherein the first conductivity typeis n-type and the second conductivity type is p-type.
 4. Thesemiconductor laser of claim 1 wherein said first, third, and fourthcladding layers are aluminum gallium arsenide and said cap layer isgallium arsenide.
 5. The semiconductor laser of claim 4 wherein saidactive layer is gallium arsenide.
 6. The semiconductor laser of claim 1wherein said mesa is a reverse mesa that is relatively narrow proximatesaid active layer and relatively wide adjacent said fourth claddinglayer.
 7. A semiconductor laser comprising:first and second opposedelectrodes; a plurality of successively disposed semiconductor layersdisposed between said first and second electrodes including an n-typefirst cladding layer and a p-type second cladding layer sandwiching anactive layer having a central light emitting region; and a pair ofopposed generally parallel facets transverse to said first and secondelectrodes for transmitting light out of said laser wherein saidplurality of layers forms a pnpn structure between said electrodesproximate each of said facets for suppressing current flow between saidelectrodes in said laser outside the central light emitting region.
 8. Asemiconductor laser comprising:a semiconductor substrate of a firstconductivity type; a semiconductor first cladding layer of the firstconductivity type disposed on said substrate; a semiconductor activelayer disposed on said first cladding layer and having a central lightemitting region of a second conductivity type opposite the firstconductivity type; a semiconductor third cladding layer disposed on saidactive layer; a semiconductor current blocking layer of the firstconductivity type disposed on said third cladding layer, said currentblocking layer including an opening extending to said third claddinglayer; a semiconductor fourth cladding layer of the second conductivitytype disposed on said current blocking layer and on said third claddinglayer in the opening in said current blocking layer; a semiconductorcontacting layer of the second conductivity type disposed on said fourthcladding layer; and first and second electrodes respectively disposed onsaid substrate and said contacting layer wherein said laser includesgenerally parallel first and second facets transverse to said first andsecond electrodes for transmitting laser light outside said laser, saidthird cladding layer is of the second conductivity type opposite thelight emitting region of said active layer and of the first conductivitytype proximate said facets and elsewhere outside the light emittingregion of said active layer.
 9. The semiconductor laser of claim 8wherein the first conductivity type is n-type and the secondconductivity type is p-type.
 10. The semiconductor laser of claim 8wherein said first, third, and fourth cladding layers are aluminumgallium arsenide and said contacting layer is gallium arsenide.
 11. Thesemiconductor laser of claim 8 wherein said active layer is galliumarsenide.
 12. A semiconductor laser comprising:first and second opposedelectrodes; a plurality of successively disposed semiconductor layersdisposed between said first and second electrodes including an n-typefirst cladding layer and a p-type second cladding layer sandwiching anactive layer having a central light emitting region; and a pair ofopposed generally parallel facets transverse to said first and secondelectrodes for transmitting light out of said laser wherein saidplurality of layers forms a pnpn structure between said electrodesproximate each of said facets outside the central light emitting regionfor suppressing current flow between said electrodes proximate each ofsaid facets outside the central light emitting region.